Most peptide hormone assays today are based on the principle of protein binding, e.g., radioimmunoassay. These methods are sensitive and precise, but they are not a measure of biological activity. Most clinical studies of circulating and tissue content of ACTH have utilized the radioimmunoassay technique. We are planning to embark on clinical studies employing a sensitive bioassay for ACTH. These studies will involve both normal and abnormal physiology. In addition, we would like to make some quantitative comparison to a standardized radioimmunoassay for ACTH. The ACTH bioassay will utilize the rat adrenocortical isolated cell suspension method which appears to be more sensitive than most ACTH radioimmunoassays. The bioassay method is currently being set up and characterized. A radioimmunoassay for ACTH will be developed at a later date.